Thousands of anti-war protesters will mount a peaceful challenge to a ban on marches outside parliament today, unless there is a last-minute compromise with the police. Hundreds of extra demonstrators are expected to swell the protest after police and Palace of Westminster authorities agreed to stop the march in Whitehall with a measure drawn up to counter Chartist agitators 150 years ago.

Plans for the protest, to coincide with parliament's first day back after the summer recess and Gordon Brown's expected statement on Iraq, were first put to the Metropolitan police two months ago and no mention of a ban was made at three friendly meetings.

Led by veteran activists including Tony Benn, musician Brian Eno and Labour NEC member Walter Wolfgang, a rally in Trafalgar Square was due to end with a march along Whitehall to Parliament Square.

But the organisers at the Stop the War Coalition (StWC) were told at a fourth meeting last week that the protest would not be allowed into Parliament Square under 19th century sessional orders passed to protect the passage of MPs and peers against radical mobs.

The change was attacked by the civil rights group Liberty in a letter to the Met's assistant commissioner, Chris Allison, which said: "This proposed restriction on peaceful protest is a disproportionate interference with the vital democratic rights of free expression and free assembly." The convenor of the StWC, Lindsey German, said yesterday: "We hope that even at this late stage the police will realise they are creating an unnecessary problem. We have had calls from pensioners, including a 78-year-old man, saying they are now coming to take part in the march because of the attempt to ban it."

The coalition said that, far from hindering legislators from reaching Westminster, the protest wanted as many as possible to be there, to resume House of Commons influence over foreign policy.

MPs also attacked the ban. John McDonnell, Labour MP for Hayes and Harlington, called it "an unacceptable assault on our civil liberties". David Howarth, Liberal Democrat MP for Cambridge, will join actor David Soul and comic Mark Thomas at the protest. He said: "The government should be listening to the people's protests, not attempting to block them."

The Metropolitan police said last night that it was still working with the StWC "to facilitate a lawful demonstration" in the relatively constricted space of Parliament Square, where three other protests marking MPs' first day back have been approved. A spokeswoman said the sessional order had been invoked "to uphold access and egress for MPs and peers in order to protect the democratic process".

"Our aim is to balance the right of the StWC to freedom of protest whilst maintaining the right of MPs and peers to conduct the business of either house. Currently there is no agreed route with the StWC, but we are still in dialogue and have made it clear that we wish to facilitate a lawful demonstration."

In a letter to the Met yesterday the StWC said close cooperation had seen 20 demonstrations pass off successfully in London, including an estimated 400,000 people marching past parliament in 2003 to protest against the visit of George Bush.